CHEMISTRY NOW: Chemistry of Light (Grades 9-12)

Objective:

Students will determine the emission spectra of metal salts.
Students will identify an unknown salt.

Introduction Notes:

CHEMISTRY NOW: Chemistry of Light

Title:

Chemistry of Light

Subject/Topic:

Emission spectra of elements

Grades:

9–12

Standards

Alignment:

Science as Inquiry

Structure and Properties of Matter

Chemical Reactions

Time Allowance:

One 50-minute period

Overview and Purpose/Objective(s)

(information, concepts to be learned):

Students will determine the emission spectra of metal salts.

Students will identify an unknown salt.

Vocabulary:

Excited State—Any energy state of an atom above its ground state. For atoms, excited state refers to energy levels of electrons higher than the ground-state energy. Excited states are not indefinitely stable, so the atom will eventually return to its ground state.

Electromagnetic radiation—Wave-like energy, as manifested by the generation of both electrical and magnetic oscillating fields as the waves propagate through space. Visible light represents only a small portion of the entire spectrum of electromagnetic radiation, which extends from high-frequency cosmic and gamma rays through X-rays, ultraviolet light, infrared radiation, and microwaves, down to very low frequency long-wavelength radio waves.

Emission spectrum—The continuous spectrum or pattern of bright lines or bands seen when the electromagnetic radiation emitted by a substance is passed into a spectrometer. The spectrum is characteristic of the emitting substance and the type of excitation to which it is subjected.

Ground State—The state of a physical system having the lowest possible potential energy. The ground state of a physical system tends to be stable unless energy is applied to it from the outside; states that are not the ground state have a tendency to revert to the ground state, giving off energy in the process.

Photon—A particle of light that acts as an individual unit of energy

Spectrometer (spectrophotometer, spectrograph, or spectroscope)—An instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials.

Spectrum—The characteristic wavelengths of electromagnetic radiation (or a portion thereof) that are emitted or absorbed by an object or substance.

Safety Concerns:

- Many of the salts used in this lab are toxic. Care should be taken not to ingest the salts. Wash hands after handling salts, use a scoopula when possible, and wear latex or vinyl gloves.

- Goggles should be worn.

- Be sure to follow proper safety procedures near open flames. Tie back hair and secure loose clothing.

- Be aware that the spectrometer is a high voltage device and fingers and other body parts should be kept away from the device.

Materials:

spectrometer

Bunsen burner

nichrome loop

methyl alcohol

various spectrum tubes

power supply

lithium chloride, LiCl

sodium chloride, NaCl

potassium chloride, KCl

calcium nitrate, Ca(NO3)2;or calcium chloride, CaCl2

strontium chloride, SrCl2

iron III chloride, FeCl3

copper II chloride, CuCl2

Activity/Engagement

(reinforcing lesson, making real-world connection)

Part 1—Class Discussion

In this lab we will be looking at the color of light released from electrons as they move from an excited state to a ground state. How does light occur and why does light have different colors? It all has to do with the amount of energy an electron gains and loses when moving between electron orbitals.

Light is the result of electrons moving from an excited state to a ground state. In the excited state, electrons have gained energy from a source outside the atom.

In this lab we will use a flame to do this. We will heat up salts that contain metal ions.

As the electrons of each metal ion absorb energy, they will move to higher orbitals within the atom. These electrons are defined as being in an excited state. Although electrons can gain energy, they do eventually drop down to their original orbital known as the ground state for the atom, and by doing so, the energy each electron has gained is released as a particle of light or photon.

The metal ions in each salt will produce specific colors in a flame. We can use this knowledge to understand what ions are in each salt.

Note: Not all salts or elements release photons when moving from an excited state to ground state.

Through years of experimentation, scientists have crafted a database of known flame tests (see Table 1). Take a look at this table. It will help students understand more about the composition of unknown salts.

Ask students how this information may be useful to engineers who make light bulbs or to astronomers who study the light from other galaxies.

Part 2

Emission Spectra Analysis:

After the teacher has set up the spectrum tubes, students will aim their spectroscope at the glowing tube. Students should record the appearance of the spectrum, distance between spectral lines, and the intensity and hue of the colors. They could create a colored sketch of the spectrum.

Flame Test of Metal Ions (to be conducted by the teacher as a demonstration):

To identify elements by the flame color, wet the tip of the nichrome loop in some methyl alcohol, then dip the loop into one of the salt samples. Place the sample into the blue portion of the Bunsen burner flame. Each metal salt will emit a distinctive color.

Do not tell students the identity of one of the metal salts and call this the unknown. Students will be challenged to identify the salt based upon their observations. Have students compare their results to the table of known flame tests below.

Have students record their observations on their data sheets.

Table 1: known flame colors:

Symbol

Element

Color

As

Arsenic

Blue

B

Boron

Bright green

Ba

Barium

Pale/Yellowish Green

Ca

Calcium

Orange to red

Cs

Cesium

Blue

Cu(I

Copper(I)

Blue

Cu(II)

Copper(II) non-halide

Green

Cu(II)

Copper(II) halide

Blue-green

Fe

Iron

Gold

In

Indium

Blue

K

Potassium

Lilac to red

Li

Lithium

Magenta to carmine

Mg

Magnesium

Bright white

Mn(II)

Manganese(II)

Yellowish green

Mo

Molybdenum

Yellowish green

Na

Sodium

Intense yellow

P

Phosphorus

Pale bluish green

Pb

Lead

Blue

Rb

Rubidium

Red to purple-red

Sb

Antimony

Pale green

Se

Selenium

Azure blue

Sr

Strontium

Crimson

Te

Tellurium

Pale green

Tl

Thallium

Pure green

Zn

Zinc

Bluish green to whitish green

Student Worksheet for Chemistry of Light

Name: __________________________________ Date: ______________

Group Hypothesis:__________________________________________________

Group Procedure:

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

__________________________________________________________________

Data Table:

Item

Color

Lithium chloride

Sodium chloride

Potassium chloride

Calcium nitrate

Strontium chloride

Iron (III) chloride

Copper (II) chloride

Lithium chloride

Unknown

Observations:

____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

Conclusion:

What is the identity of the unknown metal salt? Explain your choice.

____________________________________________________________________

____________________________________________________________________

Explain how colors are produced by the two methods used in this lab. Include the

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